Yes it is possible for jellyfish to become fossilised though this is a very
rare event.

Jellyfish are soft-bodied members of a large group of invertebrate animals
(animals which lack backbones) called Cnidaria or Coelenterata. This group
includes jellyfish which have no preservable hard parts, hydroids which may
have a jellyfish stage in their life cycle, and the better known corals which
secrete hard skeletons of calcium carbonate. All members of this group are
characterised by radial or wheel-like symmetry and this helps us to recognize
fossils of the group.

Most jellyfish live in the oceans, although a few freshwater species are known.
The oldest true jellyfish fossils are Jurassic in age (about 180 million
years old). Because jellyfish are gelatinous, and made up of more than 95 per cent
water, the fossils are really jellyfish impressions (like a footprint, for
example) made in fine muddy or limey sediment on the sea or lake floor. The
dead animal sank to the seabed or lake floor in a tranquil environment, and
was rapidly covered over and the hollow interior infilled with another
layer of fine mud or sand. In this way the shape of the body and quite a
lot of detail can be preserved as a mold. When the rock layers are split
open, the jellyfish impression is revealed.

No fossilised jellyfish are known from New Zealand, although the closely
related corals are common in some mudstones and limestones.